Previously, a conventional ball valve that includes a valve body having a flow inlet passage, a flow outlet passage, and a communication portion formed between the passages, a ball which has a flow communication passage and is installed rotatably around a reference axis in the communication portion of the valve body, and a ball seat which has an approximately cylindrical shape with a desired opening formed on a circumferential wall and is placed between the ball and the valve body, and the flow inlet passage and the flow outlet passage are opened and closed by rotating the ball around a reference axis is well known (see Patent Document 1).
Such a ball valve has a problem that the ball seat rotates together with the ball when the ball is rotated, so that the sectional area of the passage is changed.
As a ball valve preventing such a simultaneous rotation, the ball valve in which a protrusion is formed on the bottom of the ball seat and a hole into which a protrusion is fitted is formed on a ball seat support surface of the valve body is disclosed in the Patent Document 1.